Weather

Barotropy: the state of a fluid in which surfaces of constant density or temperature are coincident with surfaces of constant pressure. it is considered zero baroclinity. biosphere: the transition zone between the earth and the atmosphere within which most terrestrial life forms are found. it is considered the outer portion of the geosphere and the inner or lower portion of the atmosphere. cape: acronym for convective available potential energy. the amount of energy available to create convection, with higher values increasing the possibility for severe weather. derecho: a line of intense, widespread, and fast-moving thunderstorms that moves across a great distance. they are characterized by damaging straight-line winds over hundreds of miles. spanish for straight. downpour: a heavy rain. dry line: the boundary between the dry desert air mass of the southwest u.s. and the moist air mass from the gulf of mexico. it usually lies north-south across the central and southern high plains states during spring and summer. the passage of a dry line results in a sharp decrease in humidity, clearing skies, and a wind shift from southeasterly or south to southwesterly or west. its presence influences severe weather development in the great plains. first gust: another name for the initial wind surge observed at the surface as the result of downdrafts forming the leading edge or gust front of a thunderstorm. fog: a visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth, reducing horizontal visibility to less than 5/8 statute miles. it is created when the temperature and the dew point of the air have become the same, or nearly the same, and sufficient condensation nuclei are present. it is reported as "fg" in an observation and on the metar. friction: in meteorology, it is the turbulent resistance of the earth on the atmosphere. considered as the resistance of fluids (air and water) to the relative motion of a solid body. the amount is dependent on the size and shape of the body. green flash: a brilliant green coloration of the upper edge of the sun, occasionally seen as the sun's apparent disk is about to set below a clear horizon. isobar: the line drawn on a weather map connecting points of equal barometric pressure. low pressure system: an area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. this is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone. microbarograph: a instrument designed to continuously record a barometer's reading of very small changes in atmospheric pressure. sand: loose particles of hard, broken rock or minerals. in observing, sand is reported when particles of sand are raised to sufficient height that reduces visibility. it is reported as "sa" in an observation and on the metar. scud: low fragments of clouds, usually stratus fractus, that are unattached and below a layer of higher clouds, either nimbostratus or cumulonimbus. they are often along and behind cold fronts and gust fronts, being associated with cool moist air, such as an outflow from a thunderstorm. when observed from a distance, they are sometimes mistaken for tornadoes. snowfall: the rate at which snow falls, usually expressed in inches of snow depth over a six hour period. sunset: the daily disappearance of the sun below the western horizon as a result of the earth's rotation. in the united states, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. in great britain, the center of the sun's disk is used instead. time of sunset is calculated for mean sea level. theodolite: an optical instrument used to track the motion of a pilot balloon, or pibal, by measuring the elevation and azimuth angles. thunderstorm: produced by a cumulonimbus cloud, it is a microscale event of relatively short duration characterized by thunder, lightning, gusty surface winds, turbulence, hail, icing, precipitation, moderate to extreme up and downdrafts, and under the most severe conditions, tornadoes. tiros: a series of television infrared observation satellites that demonstrated the feasibility and capability of observing the cloud cover and weather patterns of earth from space. an experimental program, it was the first spaceborne system that allowed meteorologists to acquire information that was immediately put to use in an operational setting. the first u.s. weather satellite, tiros i, was launched on april 1, 1960, and tiros x, the last of the series, was launched on july 2, 1965. visibility: a measure of the opacity of the atmosphere, and therefore, the greatest distance one can see prominent objects with normal eyesight. the national weather service has various terms for visibility. surface visibility is the prevailing visibility determined from the usual point of observation. prevailing visibility is considered representative of visibility conditions at the station. sector visibility is the visibility in a specified direction that represents at least a 45 degree arc of the horizon circle. tower visibility is the prevailing visibility determined from the airport traffic control tower (atct) at stations that also report surface visibility. weather surveillance radar (wsr-88d): the newest generation of doppler radars, the 1988 doppler weather radar. the radar units, with help from a set of computers, show very detailed images of precipitation and other phenomena, including air motions within a storm..

Antarctic ocean: although not officially recognized as a separate ocean body, it is commonly applied to those portions of the atlantic, pacific, and indian oceans that reach the antarctic continent on their southern extremes. beaufort wind scale: a system of estimating and reporting wind speeds. it is based on the beaufort force or number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. the scale was devised by sir francis beaufort (1777-1857), hydrographer to the british royal navy. biosphere: the transition zone between the earth and the atmosphere within which most terrestrial life forms are found. it is considered the outer portion of the geosphere and the inner or lower portion of the atmosphere. boulder wind: a local name referring to an extremely strong downslope wind in the front range of the rocky mountains near boulder, colorado. chemosphere: a vaguely defined region of the upper atmosphere in which photochemical reactions take place. it includes the top of the stratosphere, all of the mesosphere, and sometimes the lower part of the thermosphere. cold: a condition marked by low or decidedly subnormal temperature. the lack of heat. cold advection: the horizontal movement of colder air into a location. contrast with warm advection. contrail: acronym for condensation trail. a cloud-like streamer or trail often seen behind aircraft flying in clear, cold, humid air. a vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere. cumulonimbus: a vertically developed cumulus cloud, often capped by an anvil-shaped cirriform cloud. also called a thunderstorm cloud, it is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail, tornadoes or strong, gusty winds. ecliptic: the sun's apparent path across the sky that tracks a circle through the celestial sphere. environment: the sum total of all the external conditions that effect an organism, community, material, or energy. hydrology: the study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water in streams, lakes, and on or below the land surface. inches of mercury (hg): the name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. one inch of mercury is equivalent to 33.86 millibars or 25.40 millimeters. first devised in 1644 by evangelista torricelli (1608-1647), an italian physicist and mathematician, to explain the fundamental principles of hydromechanics. latitude: the location north or south in reference to the equator, which is designated at zero (0) degrees. parallel lines that circle the globe both north and south of the equator. the poles are at 90° north and south latitude. mist: a collection of microscopic water droplets suspended in the atmosphere. it does not reduce visibility as much as fog and is often confused with drizzle. national centers for environmental prediction (ncep): as part of the national weather service, the centers provide timely, accurate, and continually improving worldwide forecast guidance products. some of the centers include the aviation weather center, the climate prediction center, the storm prediction center, and the tropical prediction center. formerly known as nmc. : for further information, contact the ncep, with central offices located in silver spring, maryland. radial velocity: a type of velocity that expresses motion toward or away from a given location. in doppler radar, it is the component of motion that is parallel to the radar beam. snow squall: a heavy snow shower accompanied by sudden strong winds, or a squall. temperate climate: climates with distinct winter and summer seasons, typical of regions found between the tropics of cancer and capricorn and the arctic and antarctic circles. considered the climate of the middle latitudes. veering: a clockwise shift in the wind direction in the northern hemisphere at a certain location. in the southern hemisphere, it is counterclockwise. this can either happen horizontally or vertically (with height). for example, the wind shifts from the north to the northeast to the east. it is the opposite of backing. wind shift: the term applied to a change in wind direction of 45 degrees or more, which takes place in less than 15 minutes. it may the result of a frontal passage, from katabatic winds, sea breezes, or thunderstorms, and in some instances, the change may be gradual or abrupt..

Advection: the horizontal transfer of any property in the atmosphere by the movement of air (wind). examples include heat and moisture advection. barograph: an instrument that continuously records a barometer's reading of atmospheric pressure. cape verde islands: a group of volcanic islands in the eastern atlantic ocean off the coast of west africa. a cape verde hurricane originates near here. contrail: acronym for condensation trail. a cloud-like streamer or trail often seen behind aircraft flying in clear, cold, humid air. a vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere. doldrums: located between 30 degrees north and 30 degrees south latitudes in the vicinity of the equator, this area typically has calm or light and variable winds. also a nautical term for the equatorial trough. easterlies: usually applied to the broad patterns of persistent winds with an easterly component, such as the easterly trade winds. gust: a sudden significant increase in or rapid fluctuations of wind speed. peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). the duration is usually less twenty seconds. high pressure system: an area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's rotation. this is clockwise the in northern hemisphere and counterclockwise in the southern hemisphere. it is the opposite of an area of low pressure or a cyclone. national meteorological center (nmc): now incorporated into the national centers for environmental prediction, it was the division of the national weather service that produced, processed, handled, and distributed meteorological and oceanographic information to users throughout the northern hemisphere, specifically u.s. governmental organizations. pressure: the force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface. runway visual range (rvr): it is the maximum distance at which the runway, or the specified lights or markers delineating it, can be seen from a position above a specified point on its center line. this value is normally determined by visibility sensors located alongside and higher than the center line of the runway. rvr is calculated from visibility, ambient light level, and runway light intensity. : : scattering: the process by which small particles suspended in the air diffuse a portion of the incident radiation in all directions. this is a primary reason for colors, such as blue skies, rainbows, and orange sunsets. when working with radars, this often refers to the more or less random changes in direction of radio energy. siberian express: a fierce, cold flow of air that originates in siberia, then moves into alaska and northern canada before moving southward into the united states. standard surface pressure: the measurement of one atmosphere of pressure under standard conditions. it is equivalent to 1,013.25 millibars, 29.92 inches of mercury, 760 millimeters of mercury, 14.7 pounds per square inch, or 1.033 grams per square centimeter. stratiform: clouds composed of water droplets that exhibit no or have very little vertical development. the density of the droplets often blocks sunlight, casting shadows on the earth's surface. bases of these clouds are generally no more than 6,000 feet above the ground. they are classified as low clouds, and include all varieties of stratus and stratocumulus. the opposite in type are the vertical development of cumuliform clouds. thermal low: also known as heat low, it is an area of low pressure due to the high temperatures caused by intensive heating at the surface. it tends to remain stationary over its source area, with weak cyclonic circulation. there are no fronts associated with it. an example is the low that develops over southwestern united states and northwestern mexico during the summer months. troposphere: the lowest layer of the atmosphere located between the earth's surface to approximately 11 miles (17 kilometers) into the atmosphere. characterized by clouds and weather, temperature generally decreases with increasing altitude..

Apogee: the point farthest from the earth on the moon's orbit. this term can be applied to any other body orbiting the earth, such as satellites. it is the opposite of perigee. corposant: a luminous, sporadic, and often audible, electric discharge. it occurs from objects, especially pointed ones, when the electrical field strength near their surfaces attains a value near 1000 volts per centimeter. it often occurs during stormy weather and might be seen on a ship's mast or yardarm, aircraft, lightning rods, and steeples. hydrosphere: considered as the water portion of the earth's surface. part of the geosphere. katabatic wind: a wind that is created by air flowing downhill. when this air is warm, it may be called a foehn wind, and regionally it may be known as a chinook or santa ana. when this air is cold or cool, it is called a drainage wind, and regionally it may be known as a mountain breeze or glacier wind. the opposite of an anabatic wind. minimum: the least value attained by a function, for example, temperature, pressure, or wind speed. the opposite of maximum. national climatic data center (ncdc): the agency that archives climatic data from the national oceanic and atmospheric administration as well as other climatological organizations. : for further information, contact the ncdc, located in asheville, north carolina. numerical forecasting: the use of numerical models, such as the fundamental equations of hydrodynamics subjected to observed initial conditions, to forecast the weather. these models are run on high-speed computers at the national centers for environmental prediction. : radiational cooling: the cooling of the earth's surface and the adjacent air. although it occurs primarily at night, it happens when the earth's surface suffers a net loss of heat due to outgoing radiation. snow: frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal form. it most often falls from stratiform clouds, but can fall as snow showers from cumuliform ones. it usually appears clustered into snowflakes. it is reported as "sn" in an observation and on the metar. snowpack: the amount of annual accumulation of snow at higher elevations. wave(s): in general, any pattern with some roughly identifiable periodicity in time and/or space. it is also considered as a disturbance that moves through or over the surface of the medium with speed dependent on the properties of the medium. in meteorology, this applies to atmospheric waves, such as long waves and short waves. in oceanography, this applies to waves generated by mechanical means, such as currents, turbidity, and the wind..

Calm: atmospheric conditions devoid of wind or any other air motion. in oceanic terms, it is the apparent absence of motion of the water surface when there is no wind or swell. cirrocumulus: a cirriform cloud with vertical development, appearing as a thin sheet of small white puffs which give it a rippled effect. it often creates a "mackerel sky", since the ripples may look like fish scales. sometimes it is confused with altocumulus, however, it has smaller individual masses and does not cast a shadow on other elements. it is also the least common cloud type, often forming from cirrus or cirrostratus, with which it is associated in the sky. collada: a strong, steady wind blowing from the north or northwest in the upper part of the gulf of california and from the northeast in the lower part. corona: a pastel halo around the moon or sun created by the diffraction of water droplets. the droplets in the cloud, such as cirrostratus, and the cloud layer itself must be almost perfectly uniform in order for this phenomena to occur. the color display sometimes appears to be iridescent. diurnal: pertaining to actions or events that occur during a twenty-four hour cycle or recurs every twenty-four hours. meteorological elements that are measured diurnally include clouds, precipitation, pressure, relative humidity, temperature, and wind. ecology: the study of the relationships between living organisms and their environment. forecast: a statement of expected future occurrences. weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist. macroburst: a large downburst with an outflow diameter of 2.5 miles (4 kilometers) or larger and damaging winds. national weather service (nws): a primary branch of the national oceanic and atmospheric administration, it is responsible for all aspects of observing and forecasting atmospheric conditions and their consequences, including severe weather and flood warnings. : for further information, contact the nws. palmer drought index: a long-term meteorological drought severity index produced by the noaa/usda (department of agriculture) joint agricultural weather facility. the index depicts prolonged times, as in months or years, of abnormal dryness or wetness. it responds slowly, changing little from week to week, and reflects long-term moisture runoff, recharge, and deep percolation, as well as evapotranspiration. sea spray: sometimes called salt spray, it is the drops of sea water (salt water) blown from the top of a wave. snow depth: the actual depth of snow on the ground at any instant during a storm, or after any single snowstorm or series of storms. sounding: a plot of the atmosphere, using data rom upper air or radiosonde observations. usually confined to a vertical profile of the temperatures, dew points, and winds above a fixed location. steam fog: a type of advection fog that is produced by evaporation when cool air passes over a warm wet surface and the fog rises, giving the appearance of steam. also called sea smoke when it occurs over the ocean. wind shear: the rate of wind speed or direction change with distance. vertical wind shear is the rate of change of the wind with respect to altitude. horizontal wind shear is the rate of change on a horizontal plane..

Calm: atmospheric conditions devoid of wind or any other air motion. in oceanic terms, it is the apparent absence of motion of the water surface when there is no wind or swell. carbon dioxide (co2): a heavy, colorless gas that is the fourth most abundant constituent of dry air, comprising 0.033% of the total. coriolis effect: a force per unit mass that arises solely from the earth's rotation, acting as a deflecting force. it is dependent on the latitude and the speed of the moving object. in the northern hemisphere, air is deflected to the right of its path, while in the southern hemisphere, air is deflected to the left of its path. it is greatest at the poles, north and south, and almost nonexistent at the equator. dense fog advisory: advisory issued when fog reduces visibility to 1/8 mile or less, creating possible hazardous conditions. environment: the sum total of all the external conditions that effect an organism, community, material, or energy. equatorial trough: the quasi-continuous area of low pressure between the subtropical high pressure areas in both the northern and southern hemisphere. foehn: a warm dry wind on the lee side of a mountain range, whose temperature is increased as the wind descends down the slope. it is created when air flows downhill from a high elevation, raising the temperature by adiabatic compression. classified as a katabatic wind. perihelion: the point of the earth's orbit that is nearest to the sun. although the position is part of a 21,000 year cycle, currently it occurs around january, when the earth is about 3 million miles closer to the sun than at aphelion. this term can be applied to any other celestial body in orbit around the sun. it is the opposite of aphelion. rawinsonde: an upper air observation that evaluates the winds, temperature, relative humidity, and pressure aloft by means of a balloon-attached radiosonde that is tracked by a radar or radio direction-finder. it is a radiosonde observation combined with a winds-aloft observation, called a rawin. universal time coordinate: one of several names for the twenty-four hour time which is used throughout the scientific and military communities..

Absolute humidity: a type of humidity that considers the mass of water vapor present per unit volume of space. also considered as the density of the water vapor. it is usually expressed in grams per cubic meter. air: this is considered the mixture of gases that make up the earth's atmosphere. the principal gases that compose dry air are nitrogen (n2) at 78.09%, oxygen (o2) at 20.946%, argon (a) at 0.93%, and carbon dioxide (co2) at 0.033%. one of the most important constituents of air and most important gases in meteorology is water vapor (h2o). asos: acronym for automated surface observing system. this system is a collection of automated weather instruments that collect data. it performs surface based observations from places that do not have a human observer, or that do not have an observer 24 hours a day. boiling point: the temperature at which a liquid changes to a vaporous state. the temperature at which the equilibrium vapor pressure between a liquid and its vapor is equal to the external pressure on the liquid. the boiling point of pure water at standard pressure is 100°c or 212°f. clear ice: a glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled in water droplets. the droplets spread out over an object, such as an aircraft wing's leading edge, prior to complete freezing and forms a sheet of clear ice. cumulonimbus mammatus: a portion of a cumulonimbus cloud that appears as a pouch or udder on the under surface of the cloud. although they do not cause severe weather, they often accompany storms. they may slowly vary in size, since they are an area of negative buoyancy convection, and is associated with severe turbulence in the lower sections of the cloud. depression: in meteorology, it is another name for an area of low pressure, a low, or trough. it also applies to a stage of tropical cyclone development and is known as a tropical depression to distinguish it from other synoptic features. discontinuity: comparatively large contrast in meteorological elements over a relatively small distance or period of time. in oceanography, it is the abrupt change or jump of a variable at a line or surface. drizzle: slowly falling precipitation in the form of tiny water droplets with diameters less than 0.02 inches or 0.5 millimeters. it falls from stratus clouds and is often associated with low visibility and fog. it is reported as "dz" in an observation and on the metar. fog bank: a fairly well-defined mass of fog observed in the distance. most commonly seen at sea, over a lake, or along coastal areas. indian summer: a period of abnormally warm weather in mid to late autumn with clear skies and cool nights. a first frost normally precedes this warm spell. longitude: the location east or west in reference to the prime meridian, which is designated as zero (0) degrees longitude. the distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's north and south poles. time zones are correlated to longitude. mackerel sky: the name given to cirrocumulus clouds with small vertical extent and composed of ice crystals. the rippled effect gives the appearance of fish scales. measured ceiling: a ceiling classification applied when the ceiling value has been determined by an instrument, such as a ceilometer or ceiling light, or by the known heights of unobscured portions of objects, other than natural landmarks, near the runway. microbarograph: a instrument designed to continuously record a barometer's reading of very small changes in atmospheric pressure. millibar (mb): the standard unit of measurement for atmospheric pressure used by the national weather service. one millibar is equivalent to 100 newtons per square meter. standard surface pressure is 1,013.2 millibars. pascal's law: when an external pressure is applied to any confined fluid at rest, the pressure is increased at every point in the fluid by the amount of external pressure applied. it means that the pressure of the atmosphere is exerted not only downward on the surface of an object, but also in all directions against a surface which is exposed to the atmosphere. formulated by blaise pascal (1623-1662), a french mathematician, theologian, and physicist. semi-permanent pressure systems: a relatively stable, stationary pressure-and-wind system where the pressure is predominately high or low with the changing season. they are not of a transitory nature, like migratory lows that develop from temperature and density differences. sky cover: the amount of the celestial dome that is hidden by clouds and/or obscurations. station pressure: the atmospheric pressure with respect to the station elevation. theodolite: an optical instrument used to track the motion of a pilot balloon, or pibal, by measuring the elevation and azimuth angles. x-rays: the portion of the electromagnetic spectrum that has a very short wave length. it has a wave length longer than gamma rays, yet shorter than visible light. x-rays can penetrate various thicknesses of all solids, and when absorbed by a gas, can result in ionization. : :.

Climatology: the study of climate. it includes climatic data, the analysis of the causes of the differences in climate, and the application of climatic data to the solution of specific design or operational problems. cold core thunderstorms: thunderstorms formed primarily due to steep lapse rates, especially when very cold air aloft overlies warmer surface air. condensation: the process by which water vapor undergoes a change in state from a gas to a liquid. it is the opposite physical process of evaporation. cut-off low: a closed cold core low completely removed from the primary westerly flow. cutoff lows may remain detached from the westerlies for days while exhibiting very little forward (eastward) progress. in some instances, a cutoff low may move to the west, or retrograde, opposite to the prevailing flow. it is important to note that a cutoff low is a closed low, but not all closed lows are cutoff lows. easterly wave: an inverted, migratory wave-like disturbance or trough in the tropical region that moves from east to west, generally creating only a shift in winds and rain. the low level convergence and associated convective weather occur on the eastern side of the wave axis. normally, it moves slower than the atmospheric current in which it is embedded and is considered a weak trough of low pressure. it is often associated with possible tropical cyclone development and is also known as a tropical wave. echo: the energy return of a radar signal after it has hit the target. fetch: an area of the water surface over which waves are generated by a wind having a constant direction and speed. also, it is the name given to the length of the fetch area, measured in the direction of the wind from which the seas are generated. one of the ingredients for lake effect snow is the fetch of the water over which cold air can gain moisture. heat balance: the equilibrium which exists on the average between the radiation received by the earth and atmosphere from the sun and that emitted by the earth and atmosphere. the balance between heat loss (long wave radiation from the earth back into the atmosphere) and heat gain (incoming solar radiation). icelandic low: a semi-permanent, subpolar area of low pressure in the north atlantic ocean. because of its broad area and range of central pressure, it is an area where migratory lows tend to slow down and deepen. it is strongest during a northern hemisphere winter and early spring, centered over iceland and southern greenland, and is the dominate weather feature in the area. during the summer, it is weaker, less intense, and might divide into two parts, one west of iceland, the other over the davis strait between greenland and baffin island. then the azores or bermuda high becomes the dominate weather feature in the north atlantic. inches of mercury (hg): the name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. one inch of mercury is equivalent to 33.86 millibars or 25.40 millimeters. first devised in 1644 by evangelista torricelli (1608-1647), an italian physicist and mathematician, to explain the fundamental principles of hydromechanics. metar: acronym for meteorological aerodrome report. it is the primary observation code used in the united states to satisfy requirements for reporting surface meteorological data. minimum reporting requirements includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting. microbarograph: a instrument designed to continuously record a barometer's reading of very small changes in atmospheric pressure. monsoon: the seasonal shift of winds created by the great annual temperature variation that occurs over large land areas in contrast with associated ocean surfaces. the monsoon is associated primarily with the moisture and copious rains that arrive with the southwest flow across southern india. the name is derived from the word mausim, arabic for season. this pattern is most evident on the southern and eastern sides of asia, although it does occur elsewhere, such as in the southwestern united states. pressure tendency: the pressure characteristic and amount of pressure change during a specified time period, usually the three hour period preceding the observation. satellite images: images taken by a weather satellite that reveal information, such as the flow of water vapor, the movement of frontal system, and the development of a tropical system. looping individual images aids meteorologists in forecasting. one way a picture can be taken is as a visible shot, that is best during times of visible light (daylight). another way is as an ir (infrared) shot, that reveals cloud temperatures and can be used day or night. snowburn: a burn of the skin, like a sunburn, but caused by the sun's rays reflected off the snow surface. snow creep: a continuous, extremely slow, downhill movement of a layer of snow. station elevation: the vertical distance above mean sea level that is the reference level for all current measurements of atmospheric pressure at that station. sublimation: the process of a solid (ice) changing directly into a gas (water vapor), or water vapor changing directly into ice, at the same temperature, without ever going through the liquid state (water). the opposite of crystallization. synoptic scale: the size of migratory high and low pressure systems in the lower troposphere that cover a horizontal area of several hundred miles or more. vapor trail: a cloudlike streamer or trail often seen behind aircraft flying in clear, cold, humid air. a vapor trail is created when the water vapor from the engine exhaust gases are added to the atmosphere. also called a contrail, for condensation trail. wave length: the least distance between particles moving in the same phase of oscillation of a wave. in oceanography, it is the horizontal distance between the highest parts of two successive wave crests above the still water level, separated by a trough that is below the still water level, and it is measured in meters. windward: the direction from which the wind is blowing. also the upwind side of an object. the opposite of the downwind or leeward side..

Beaufort wind scale: a system of estimating and reporting wind speeds. it is based on the beaufort force or number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. the scale was devised by sir francis beaufort (1777-1857), hydrographer to the british royal navy. continental air mass: an air mass with continental characteristics. it is a secondary characteristic of an air mass classification, signified by the small "c" before the primary characteristic, which is based on source region. for example, cp is an air mass that is continental polar in nature. cyclonic flow: winds that blow in and around a cyclone, that is counterclockwise in the northern hemisphere, clockwise in the southern hemisphere. diffluence: a rate at which wind flow spreads apart along an axis oriented normal to the flow in question. the opposite of confluence. diffraction: the result of light waves interfering with other after passing through a narrow aperture, causing them to bend or spread. downpour: a heavy rain. dry line: the boundary between the dry desert air mass of the southwest u.s. and the moist air mass from the gulf of mexico. it usually lies north-south across the central and southern high plains states during spring and summer. the passage of a dry line results in a sharp decrease in humidity, clearing skies, and a wind shift from southeasterly or south to southwesterly or west. its presence influences severe weather development in the great plains. geophysics: the study of the physics or nature of the earth and its environment. it deals with the composition and physical phenomena of the earth and its liquid and gaseous envelopes. areas of studies include the atmospheric sciences and meteorology, geology, seismology, and volcanology, and oceanography and related marine sciences, such as hydrology. by extension, it often includes astronomy and the related astro-sciences. hydrology: the study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water in streams, lakes, and on or below the land surface. inches of mercury (hg): the name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. one inch of mercury is equivalent to 33.86 millibars or 25.40 millimeters. first devised in 1644 by evangelista torricelli (1608-1647), an italian physicist and mathematician, to explain the fundamental principles of hydromechanics. jet stream: an area of strong winds that are concentrated in a relatively narrow band in the upper troposphere of the middle latitudes and subtropical regions of the northern and southern hemispheres. flowing in a semi-continuous band around the globe from west to east, it is caused by the changes in air temperature where the cold polar air moving towards the equator meets the warmer equatorial air moving polarward. it is marked by a concentration of isotherms and strong vertical shear. lightning: a sudden and visible discharge of electricity produced in response to the build up of electrical potential between cloud and ground, between clouds, within a single cloud, or between a cloud and surrounding air. metar: acronym for meteorological aerodrome report. it is the primary observation code used in the united states to satisfy requirements for reporting surface meteorological data. minimum reporting requirements includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting. millibar (mb): the standard unit of measurement for atmospheric pressure used by the national weather service. one millibar is equivalent to 100 newtons per square meter. standard surface pressure is 1,013.2 millibars. refraction: the bending of light or radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature. relative vorticity: the sum of the rotation of an air parcel about the axis of the pressure system and the rotation of the parcel about its own axis. satellite: any object that orbits a celestial body, such as a moon. however, the term is often used in reference to the manufactured objects that orbit the earth, either in a geostationary or a polar manner. some of the information that is gathered by weather satellites, such as goes9, includes upper air temperatures and humidity, recording the temperatures of cloud tops, land, and ocean, monitoring the movement of clouds to determine upper level wind speeds, tracing the movement of water vapor, monitoring the sun and solar activity, and relaying data from weather instruments around the world. snow garland: snow appearing as a beautiful long thick rope draped on trees, fences and other objects. formed by the surface tension of thin films of water bonding individual snow crystals. snow grains: frozen precipitation in the form of very small, white, opaque grains of ice. the solid equivalent of drizzle. it is reported as "sg" in an observation and on the metar. snow roller: the product of moist, cohesive snow that when initiated by wind rolls across the landscape, gathering snow until it can no longer move. it is shaped like a rolled sleeping bag, some reaching four feet across and seven feet in diameter. thermosphere: a thermal classification, it is the layer of the atmosphere located between the mesosphere and outer space. it is a region of steadily increasing temperature with altitude, and includes all of the exosphere and most, if not all, of the ionosphere. troposphere: the lowest layer of the atmosphere located between the earth's surface to approximately 11 miles (17 kilometers) into the atmosphere. characterized by clouds and weather, temperature generally decreases with increasing altitude. unstable/ instability: occurs when a rising air parcel becomes less dense than the surrounding air. since its temperature will not cool as rapidly as the surrounding environment, it will continue to rise on its own. : one side of the walker circulation is associated with rising motion, clouds, and rain while the opposite side with sinking motion, generally fair weather and little or no rain. this circulation is large in spatial extend normally extending anywhere from way around the world or completely around the world (thus either 1 or 2 upward and 1 or 2 downward branches to the circulation). whiteout: when visibility is near zero due to blizzard conditions or occurs on sunless days when clouds and surface snow seem to blend, erasing the horizon and creating a completely white vista..

Aleutian low: a semi-permanent, subpolar area of low pressure located in the gulf of alaska near the aleutian islands. it is a generating area for storms and migratory lows often reach maximum intensity in this area. it is most active during the late fall to late spring. during the summer, it is weaker, retreating towards the north pole and becoming almost nonexistent. during this time, the north pacific high pressure system dominates. arctic jet: the jet stream that is situated high in the stratosphere in and around the arctic or antarctic circles. it marks the boundary of polar and arctic air masses. bright band: a narrow, intense radar echo due to water-covered ice particles at the melting level where reflectivity is at its greatest. cumulus: one of the three basic cloud forms (the others are cirrus and stratus). it is also one of the two low cloud types. a cloud that develops in a vertical direction from the base (bottom) up. they have flat bases and dome- or cauliflower-shaped upper surfaces. the base of the cloud is often no more than 3,000 feet above the ground, but the top often varies in height. small, separate cumulus are associated with fair weather (cumulus humilis). with additional heating from the earth's surface, they can grow vertically throughout the day. the top of such a cloud can easily reach 20,000 or more into the troposphere. under certain atmospheric conditions, these clouds can develop into larger clouds, known as towering cumulus (cumulus congestus), and may produce a rain shower. further development may create a cumulonimbus. dusk: the period of waning light from the time of sunset to dark. geostationary satellite: an orbiting weather satellite that maintains the same position over the equator during the earth's rotation. also known as goes, an acronym for geostationary operational environmental satellite. muggy: a subjective term for warm and excessively humid weather. peak gust: the highest instantaneous wind speed observed or recorded. saturation point: the point when the water vapor in the atmosphere is at its maximum level for the existing temperature. snowburn: a burn of the skin, like a sunburn, but caused by the sun's rays reflected off the snow surface. snow line: the lowest elevation area of a perennial snow field on high terrain, such as a mountain range. tilt: the inclination to the vertical of a significant feature of the pressure pattern or of the field of moisture or temperature. for example, midlatitide troughs tend to display a westward tilt with altitude through the troposphere. wet bulb depression: dependent on the temperature and the humidity of the air, it is the difference between the dry bulb and the wet bulb readings. wet bulb thermometer: a thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin-covered bulb of a thermometer. the wet bulb temperature is used to compute dew point and relative humidity. one of the two thermometers that make up a psychrometer.

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